A highly simplified picture of the Earth’s structure would be that it comprises a molten metallic core, a solid ‘rock’ shell (the mantle) enclosing it and a thin veneer of crust enveloping it. The temperature of the core is 6,000o C or so, much higher than outside it, so it is losing heat all the time, although at an extremely slow rate. This cooling of the core brings out heat, via the mantle and the crust, to the surface of the Earth.
The effect of this is to create a rise in temperature as we go deeper into the earth. In the crust, most places have a nominal geothermal gradient of around 15o C per km. It is this warmth coming from below that stops the ground freezing up too much, as gardeners among us will have observed.
However, in geologically unstable places, for example where there is volcanic activity, or deep faults in the crust, the hot mantle material is brought closer to the surface increasing the temperature gradient. So in places like Iceland, and areas like Lardorello in Italy, the surface temperature get high enough to have steam coming out of the ground. In such places, if you drill boreholes just a km or two deep, you can get steam to generate electricity and there are successful geothermal power plants utilising this in many parts of the world.
The British Isles are geologically stable: we are not exposed to eruptions of volcanoes or violent earthquakes. By the same token, we are not ideally placed for geothermal power generation either. However, there are areas of Britain that have much higher than average temperature gradients, and therefore higher temperatures closer to the ground. One such area is Cornwall, famous for mining over centuries. Miners have always known that temperatures underground are always warmer than out side, but Cornish miners recorded debilitatingly high temperatures in some of the mines, like South Crofty in Camborne.
Part of the search for alternative energy sources—first started when the oil prices soared in the early Seventies (see Alternative Energy, Around Langley October 2020)—included our research group at Imperial College looking at why Cornwall was so ‘hot’. Geologists knew that the whole peninsula was a big granite structure, covered thinly by other (sedimentary) rock, but with the granite exposed at the surface in several places along its length. Measurements in a series of boreholes in Cornwall showed that the temperature gradient could be 30o C/km or so (compared to the more usual 15oC/km). No wonder the miners were sweltering working underground.
The reason for the increased heatflow in the area was easily identified as the radioactivity of granites. The rate of heat generation by radioactivity is minute. However, several kilometres thickness of granite will slowly add to the heat coming up from the core, eventually doubling the temperature gradient. If there is a light sedimentary cover over the granite, there is a ‘blanket effect’ and the subsurface temperatures are even higher.
What our modelling predicted was that at 2 km depth in parts of Cornwall, the temperature would be about 80oC: not hot enough for power generation, but excellent for ‘space heating’. That is, for heating homes, greenhouses, fish farming and so forth. The test boreholes produced water at that temperature for decades.
At 6 km the predicted temperatures were over 200o C, which is enough for electricity generation. However, the viability of this could not be verified, because the research funding was cut at that point. The little research undertaken by the EU (then EEC) was channelled into a project in Soultz, in France.
Another source of geothermal energy, although ‘low grade’, is in areas where there are water-bearing structures, or aquifers underground, in sedimentary rock formations. A leisure centre and civic buildings in Southampton were heated by hot water from sedimentary rocks for many years. And the scheme has been expanded to include other uses, and also to pre-heat water used in other powerproduction.
You may well be asking yourself: what about the other granites in the UK; and how do you get hot water from a dry rock such as granite. That discussion is for another day.
But why are we not exploiting this renewable heat source? The answers to that is easy: politics and economics! When the drilling costs are comparable to production costs of other sources, the politicians may even turn to low grade geothermal energy! In the meanwhile, we can continue to ‘sit on’ this free source of energy—it will be there when we decide to use it!
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